In recent years, of the types of Digital Versatile Disks (DVDs), a recording-type optical disk has been prevalent as the optical-information recording medium, for example DVD-R or DVD-RW (below also called “DVD-R/RW” as a whole), in which wobbling for causing a zigzag movement on the guiding grooves to be the information-recording track is provided at the time of manufacturing the disks in order to accurately detect the corresponding linear velocities at the respective radial positions. In such a case, a format is adopted such that the wobble frequency is set to be constant when performing CLV (Constant Linear Velocity) rotational control. Thus, in an optical-disk apparatus using the optical disk as described above, a signal based on the wobbling on the guiding grooves (called “a wobble signal”) is detected so as to control rotation of the optical disk and to generate a clock for recording.
Furthermore, in compact disks such as CD-R and CR-RW, for example, address (time) information called ATIP is provided using frequency modulation, and in other DVDs such as DVD+R and DVD+RW, address information called ADIP is recorded in the wobble signal using phase modulation.
However, in the case of the DVD-R/RW as described above, the wobble signal does not contain information within the disk face, and instead pre-pits which contain address information are formed in a track (a land), for guiding, between two tracks (guiding grooves). A wobble signal is a signal generated at a constant periodicity with a relatively small amplitude, whereas a pre-pit (LPP: Land Pre-Pit) signal is a signal generated intermittently with a relatively large amplitude, so that basically both are separable.
The pre-pit signal is obtained by binarizing with a predetermined slice level a signal based on a push-pull signal detected with an optical-detecting device having at least two optical-detecting regions of an optical pickup. However, the maximum-amplitude value of the pre-pit signal may vary depending on the optical-axial offset of an object lens of the optical-disk apparatus, the tilt of the disk, a variation in the reflectance within the disk face, etc.
Therefore, when fixing the slice level so as to binarize the push-pull signal containing the pre-pit signal, it may lead to erroneously detecting the pre-pit signal. More specifically, in a prerecorded region within the disk face, it is important to set the slice level more appropriately since the crosstalk noise from a RF signal of the recording data leads to a likelihood of erroneously detecting the pre-pit signal.
Thus, in the related art, a method of appropriately setting the slice level is proposed as follows.
For instance, in Patent Document 1, a pre-pit signal-detecting circuit is disclosed in which a maximum-amplitude value of the pre-pit signal is detected so as to potentially-divide the detected value for setting the slice level.
Patent Document 1
JP2000-207744A
However, with such related-art method, as the wobble signal itself, which may be a cause of the erroneous detection, is not detected and a comparison with the wobble signal is not performed when setting the slice level, there exist a problem in which the likelihood of the erroneous detection may remain.